Why pensions of pensionnaires in some countries, including mine, are reduced by up to 50% by law if they keep working?

Pensions are often too small to survive on or to live decently. Many pensionnaires are forced to keep working to earn their living. Yet, if they do it legally, their pension are automatically reduced. Why? Old people earned their pensions. It's unfair from the part of the Government to take money away from poor people who earned it.

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What if lawmakers let pensionnaires keep their pensions regardless them them having other sources of income?